Many optical members and devices are required to be free of internal inclusions. For example, the optical devices used in modern photolithographic processes and equipment for the production of semiconductor products must be free of inclusions. Such inclusions could be small gas bubbles, refractory particles and platinum particles and the like introduced into the glass during the preparation process. The inclusions, in micron size or submicron size, are not acceptable in stepper lenses and photomask substrates. It is important that such inclusions be detected in the inspection process before a piece of glass bulk material is processed into the lens or photomask substrate. Inclusion detection in other transparent media, such as in bulk plastic material, for use as window panes, visors, or optical members, is also necessary for many applications.
However, detecting small (micron scale and submicron) inclusions in solid media, such as glass, has been a challenge. The difficulties associated with various practices are sensitivity, resolution, depth of focus, to name a few. Microscopy has the capability to detect inclusion down to the submicron range, yet it has an extremely narrow depth of focus and a small sampling area at high magnification. If used alone, these restrictions make it next to impossible to analyze bulk glass. Diffused reflection/scattering has been used to identify inclusion. After mapping their location, the inclusion can be further determined by microscopy. Nevertheless, the detection limit for the diffused reflection/scattering approach is about 5 microns and as low as 1 μm. In addition, the thickness of the glass is again somewhat restricted by the narrow depth of focus of the microscopy technique.
Small particles suspended in a fluid media, such as a liquid or gas, on the other hand, can be measured routinely by light scattering techniques. The differences between inclusions in a solid glass and particles suspended in a fluid are critical. One difference is that an inclusion in a glass is stationary. Its concentration level is normally very low, thus the signal intensity is so weak that it can hardly be distinguished from noise. Noise is the detected light that is not generated by scattering and/or reflection of the inclusion. In addition, the location of inclusions in glass would be valuable information. Due to the dynamic nature of the suspended particles in a fluid media, their location cannot be mapped.
U.S. Pat. No. 6,388,745 B2 and U.S. Patent Application Publication No. 2001/0040678, which are relied upon and incorporated herein by reference in their entirety, disclose an apparatus and a process for detecting inclusions in transparent sheets such as glass sheets. The apparatus includes a light source providing a primary light beam, a lens for focusing a majority of the scattered light generated by the inclusion, and a CCD array detector for detecting the focused signals. The lens has a light trap which blocks the primary light beam and prevents it from entering the detector to improve signal-to-noise ratio.
U.S. Pat. No. 6,404,489 B1, which is relied upon and incorporated herein by reference in its entirety, discloses another apparatus and process for detecting inclusions in transparent sheet. The apparatus comprises a laser source providing a primary collimated laser beam, at least one light trap positioned on an exterior surface of the sheet to be inspected and at least one detector for detecting the scattered light signals generated by the inclusions. The light trap blocks the primary laser beam and prevents illuminating the surface contaminants of the glass sheet. In one embodiment as disclosed in this patent reference, two light traps are used. The light detector as disclose in this reference can be a two-dimensional CCD array.
In general, the devices and methods in the above references were to be used for inspection of transparent glass articles having precision surfaces with a low rate of surface defects. The inspection light beam used in these devices enters into the substrates through the side surfaces without appreciable reflection loss and scattering. Moreover, the very few surface defects do not cause significant interference to the inspection of internal inclusions. For substrates having a large number of surface defects, these processes and instruments are not suitable. An example of substrates having a large number of surface defects are those lapped but not precision polished. Those substrates may have a high surface roughness such that when placed in the air, they appear diffuse because of light scattering at the surfaces. If the processes and instruments disclosed in the prior art references are used directly on these substrates, the collimated inspection light beam will be scattered significantly by the surface defects and the light scattering signals generated by internal inclusions will be drowned.
Of course, one way to inspect the internal inclusions of a substrate having large number of surface defects is to first precision polish the surfaces before inspection. This, however, is not always feasible. Besides, if internal inclusions can be detected and located before surface polishing of the substrate, substantial cost savings can be effected by avoiding the costly polishing of a defective product.
Therefore, there remains a genuine need of a method and an apparatus for inspecting the internal inclusions of substrates having considerable amounts of surface defects without the need of precision surface polishing.
The present invention satisfies this need.